Conditional access systems are well known and widely used in conjunction with currently available pay television systems. At present, such systems are based on the transmission of programmes scrambled with control words which are received by subscribers having a set-top box and a smart card for each subscription package. The smart card for a subscription package from a particular service provider allows the scrambled programmes within the package to be descrambled and viewed. The broadcast stream further contains entitlement management messages and entitlement control messages, which are necessary for the smart card to descramble the broadcast. The terms scrambled and encrypted are used interchangeably in this application. WO 98/43426 discloses a digital satellite television system in which the entitlement management messages are transmissible to the set-top box via a modem based back channel, rather than via the broadcast channel. This is done to speed up viewer authorisation in viewing systems such as pay-per-view, since there is a certain time delay before the subscriber authorisation system (SAS) can include the proper entitlement management messages in the broadcast stream. However, the entitlement control messages which contain the control word in an encrypted format are sent via the broadcast channel. The control word is decrypted at the set-top box by means of a smart card.